Lng expander cycle process employing integrated cryogenic purification

ABSTRACT

A process and apparatus for the liquefaction of natural gas wherein raw feedstock is cryogenically fractionated to remove essentially all of the carbon dioxide and C5 hydrocarbons therefrom, and wherein the cryogenically purified feedstock is cooled and liquefied under pressure in a cryogenic heat exchanger. The pressurized cold liquid from the heat exchanger is isenthalpically expanded to reduce the pressure and further cool the liquid while at the same time flashing a minor gas fraction. Refrigeration for the liquefaction of the natural gas is supplied by a circulating refrigerant stream which is compressed and workexpanded to obtain the necessary cooling. The minor flash gas portion of the liquefaction step is commingled with the circulating refrigerant stream so that the analysis of the refrigerant stream is always rich in the lighter portions of the liquefaction stream, thus aiding in maintaining refrigeration temperature differentials to drive the liquefaction step. The work-expanded refrigerant portion undergoes a compression cycle and is work-expanded in a series of expansion turbines. The expansion turbines furnish at least part of the power necessary to drive the compressor system in the refrigerant gas cycle.

United States Patent [191 Pachaly Apr. 3, 1973 Robert W. Paclialy,Greenwich, Conn.

[75] Inventor:

[73] Assignee: Gulf Research & Development Company, Pittsburgh, Pa.

22 Filed: Apr. 20, 1971 21 Appl.No.: 135,615

[52] US. Cl. ..62/39, 62/26, 62/20, 62/28, 62/40 [51] Int. Cl ..F25j1/00, F25j 3/00, F25j 3/02 [58] Field of Search ..62/23, 24, 27, 28, 26,29, 62/38, 39, 40, 54, 52, 53, 30, 40

[56] References Cited UNITED STATES PATENTS 3,675,435 7/1972 Jackson..62/28 3,292,381 12/1966 Bludworth 2,600,110 6/1952 Hachmuth 3,348,38410/1967 Harmens ..62/38 Primary Examiner-Norman Yudkoff AssistantExaminer-Arthur F. Purcell Attorney-Meyer Neishloss, Deane E. Keith andThomas G. Ryder FUEL 6,45

CRUDE FEED 6A5 GZ YCOL ,EXMNDER [57] ABSTRACT A process and apparatusfor the liquefaction of natural gas wherein raw feedstock iscryogenically fractionated to remove essentially all of the carbondioxide and C hydrocarbons therefrom, and wherein the cryogenicallypurified feedstock is cooled and liquefied under pressure in a cryogenicheat exchanger. The pressurized cold liquid from the heat exchanger isisenthalpically expanded to reduce the pressure and further cool theliquid while at the same time flashing a minor gas fraction.Refrigeration for the liquefaction of the natural gas is supplied by acirculating refrigerant stream which is compressed and work-expanded toobtain the necessary cooling. The minor flash gas portion of theliquefaction step is commingled with the circulating refrigerant streamso that the analysis of the refrigerant stream is always rich in thelighter portions of the liquefaction stream, thus aiding in maintainingrefrigeration temperature differentials to drive the liquefaction step.The work-expanded refrigerant portion undergoes a compression cycle andis work-expanded in a series of expansion turbines. The expansionturbines furnish at least part of the power necessary to drive thecompressor system in the refrigerant gas cycle.

17 Claims, 2 Drawing Figures [XPANDEA LNG EXPANDER CYCLE PROCESSEMPLOYING INTEGRATED CRYOGENIC PURIFICATION RELATED APPLICATIONS Thisapplication is related to Ser. No. 36,277, filed May 1 l, 1970 andentitled Apparatus and Process for Liquefaction of Natural Gases.

BACKGROUND OF THE INVENTION This invention relates to the liquefactionof a gas; and, more particularly, to a method and apparatus for theliquefaction of natural gas. Even more particularly, this inventionrelates to a LNG expander cycle process employing an integratedcryogenic purification, and to apparatus therefor.

Natural gas is mostly composed of methane, but usually contains smallamounts of heavier hydrocarbons such as ethane, propane, butane and thelike as well as small amounts of aromatic hydrocarbons. Natural gas alsocontains minor amounts of non-hydrocarbons, such as water, nitrogen,carbon dioxide and the like. Thus, the present invention is directed toa cryogenic purification of raw natural gas to remove substantially allof the water, carbon dioxide and heavy hydrocarbons therefrom, and tothe subsequent liquefaction of the purified natural gas to form a liquidproduct stream comprised mainly of methane and substantially free fromcarbon dioxide and heavy hydrocarbons.

Recently, there has been an increasing demand for a simple yet economicprocess for reducing natural gas to a liquified state. One of thereasons is that natural gas wells are being discovered in remote partsof the world where transporting the gas to the point of consumption bypipeline is difficult or impossible. Transportation of natural gas inthe gaseous state by marine vessels would be uneconomical, unless thegaseous materials were compressed, and this in turn would be impracticalsince the containers would have to be exceptionally strong and extremelylarge to hold the gas to be transported. The cost of such containers andthe industrial hazards that attend their use are so great that thetransportation of compressed gas is impractical.

Accordingly, an efficient process for the liquefaction of natural gas isof great importance, particularly,

where the supply thereof is in a remote area and there 7 is a demand ata distant market place. Such a process is particularly important wherethe gas must be transported by marine vessels since, by liquefaction ofnatural gas, the volume thereof can be reduced to nearly onesix-hundredth its volume and the containers need not be of thethickness, strength, and capacity necessa ry for the shipment ofcompressed gas.

DESCRIPTION OF THE PRIOR ART derdeveloped and remote areas of the world.For example, a system that requires molecular sieves and/or specificadsorbents for the initial purification of raw natural gas feedstock isless economically operated in areas where such sieves and adsorbents arenot readily available and/or where spent sieves and adsorbants haveessentially no salvage value. Still other prior art system requirecomplicated high pressure equipment which is difficult to maintain andto control automatically. And yet others require the use of expensiverefrigerants which must be shipped to the liquifying plant.

At present, there are three known basic cycles for liquefaction ofnatural gases. These are generally referred to as the Cascade Cycle,Multicomponent Refrigerant Cycle and Expander Cycle. Many minorvariations can be effected in the design of each type of cycle to adaptit to the specific process requirements.

Briefly, the Cascade Cycle consists of a series of heat exchanges withthe feed gas, each exchange being at successively lower temperatureuntil the desired liquefaction is accomplished. The levels ofrefrigeration are obtained with different refrigerants or with the samerefrigerant at different evaporating pressures. Frequently, acombination of both approaches is used. The high efficiency of theCascade Cycle is offset by rather high investment cost in the extensiveheat exchange and compression systems. Piping costs are high andconsiderable area is necessary for the large amount of equipment. Thistype of plant is more easily justified when fuel costs are high such asat the delivery end of a pipeline system. This-system would beimpractical, however, where there is limited space or poor soilconditions as exists in many of the remotely located gas wellsdiscovered in the world.

A Multicomponent' Refrigerant Cycle has been operated on a pilot scalebut no large commercial operation has been achieved to date, althoughthe basic design is known to those skilled in the art. This process wasdesigned in an effort to eliminate some of the complexity of the CascadeCycle and still retain the low power requirement. The process uses acarefully controlled analysis of the hydrocarbon refrigerant stream sothat at successive temperature levels of cooling specific liquidfractions are obtained. Each of these liquid fractions, condensed athigh pressure, is then evaporated to obtain the required refrigeration.The composition changes of the successively condensed fractions whenevaporated at a constant pressure furnish distinct temperature levels ofheat exchange. In this respect, the system operates as an autocascadecycle,

The compression system of the foregoing plant is simple since only onerefrigerant is used. However, the heat exchange system and its controlsare extensive and costly. The nature of the equipment requires seriesflow of all refrigerant gas over the cascade type of exchangers. Thiscan be accomplished by vertically stacking the exchangers to a height ofover 200 feet. Unless good soil bearing is available, the support ofsuch a unit is difficult and the erection of such unit on the deck of afloating vessel presents serious problems of stability. To segmentizethe heat exchangers would require very large vapor lines in the range of5 to 6 feet in diameter.

Therefore, this type of cycle would be impractical in a liquefactionplant near off-shore wells, where the soil bearing is poor or where thewells are in a nonindustralized part of the world. A largeMulticomponent Refrigerant cycle also requires a complete hydrocarbonfractionating unit to prepare the pure components required to maintainthe carefully controlled refrigerant analysis.

The Expander Cycle is similar to that used on most of the large airseparation plants today and it does have the advantage of simplicityover a Cascade Cycle. in this cycle, gas is compressed to a selectedpressure, cooled, then allowed to expand through an expansion turbine,thereby performing work and reducing the temperature of the gas. it isquite possible to liquify a portion of the gas in such an expansion. Thelow temperature gas is then heat exchanged to effect liquefaction of thefeed. The power obtained from the expansion is usually used to supplypart of the main compression power utilized in the refrigeration cycle.If the expander cycle is a closed cycle, any suitable refrigerant gascan be used. If it is an open cycle liquid natural gas plant, therefrigerant would have to be methane or a methane-nitrogen mixture asthis would be flashed from the gas-liquid separator in the process.

An expander cycle plant is compact, has minimum items of equipment,simple control and utilizes all standard machinery and heat exchangers.This type of plant has an important added advantage of mechanicalsimplicity that is particularly significant when considering operationsin remote areas of the world.

An efficient Expander Cycle method for liquefaction of low boiling gasessuch as oxygen and nitrogen is presently known. The heat exchanger cycleof this process is operated under 400 to 1000 psia pressure and coolingis made more efficient by causing components of the warming stream toundergo a plurality of work expansion steps with intervening reheating.In this process, two or more heat exchangers are employed in series withan intermittent refrigeration of the incoming gas. A portion of the feedstream which had been previously cooled by a warm-leg heat exchanger anda refrigeration unit is work expanded and thereafter used to absorb heatfrom the remaining portion of the feed stream in a countercurrent heatexchanger. The warmed effluent gas from the heat exchanger isworkexpanded a second time and this cooled and expanded gas is combinedwith the flash gas to be used to absorb heat from the incoming feedstream in the second heat exchanger. The cold effluent from the secondheat exchanger is isenthalpically expanded and passed to a gas-liquidseparator to remove the liquid for storage and the flash gas is combinedwith the work-expanded gas as discussed above. The combined warmedeffluent gas from the heat exchangers is recycled to the feed stream tobe recompr'essed and undergoes the foregoing liquefaction.

This type of process suffers the drawback of expensive refrigerants andseparate compression and expansion systems driven by an outside sourceof power to maintain its operation. Furthermore, such a process is notpracticable in the liquefaction of natural gas since there is noprovision for handling the heavy gases which freeze at the temperaturesencountered in the heat exchangers. In addition, if the flash gas wasrecycled back to the feed stream, the lower boiling ends, i.e.,nitrogen, etc. of the feed mixture would increase in the heat exchangerliquifier causing an imbalance in the system requiring additional energyto liquify and cause thermodynamic inefficiency. This latter problem isnot apparent when there is only one pure material to be liquified suchas nitrogen and oxygen, but when dealing with the liquefaction ofnatural gases which contain a plurality of gases having boiling pointslower than methane, the problem is paramount.

In the above-mentioned related application, Ser. No. 36,277, filed May11, 1970, it is pointed out that the Expander Cycle process can besuccessfully utilized in liquifying natural gas by separating a combinedresidual flash gas and refrigerant gas effluent from the heatexchanger-liquifier into a bleed portion and a refrigerant gas portion,in stead of recycling this combined mixture to the feed stream asdescribed in the heretofore mentioned prior art. While this latterprocess offers significant advantages over the previously describedprior art processes, it requires the use of molecular sieves andadsorbents to purify the raw feed gas before subjecting it toliquefaction. Accordingly, the use of this process in a remote part ofthe world would require periodic shipments of fresh sieves andadsorbents to the plant site, as well as shipments of the spent sievesand adsorbents to an industralized area for salvage.

In light of the inherent problems associated with the production of.liquified natural gas, the foregoing prior art processes do not offer asufficiently economic and practicable system for liquifying natural gasin remote non-industrial parts of the world where the soil bearing ispoor and where expended materials have little or no salvage value.

SUMMARY OF THE INVENTION It has now been discovered that in an ExpansionCycle process, a greater proportion of the natural gas feed is liquifiedefficiently and a substantially selfbalancing process is obtained when araw natural gas feedstock is purified by an integrated cryogenicfractionation prior to liquefaction andwhen the liquefaction comprisesseparating the combined residual flash gas and refrigerant gas used toliquify and subcool the natural gas into a bleed gas portion and arecycle refrigerant gas portion instead of recycling the entire of theflash and refrigerant gases.

Since the liquefaction apparatus and process of the present invention iscompact, efficient and self-sustaining, it is capable of beingprefabricated on a mobile platform which may be of a marine type thatcan be shipped to distant and remote parts of the world. The mobilemarine means contemplated by the present invention can be of the typedisclosed in the US. Pat. No. 3,161,492 to Keith et al., whichdisclosure is incorporated herein by reference. Additionally, the mobilemarine platform can be of the floating type wherein the platform is notsupported by mechanical means, such as caissons, engaging the marinefloor but rather is supported by the buoyancy of the platform itself.Such a floating platform can take the form of a barge or conventional,self-powered ocean-going vessel such as a tanker. Such an apparatus andprocess prefabricated on a marine means is so designed as to permitdirect loading of the liquid into a transport means such as a ship ortanker. Furthermore, by employing the apparatus of the present inventionit is possible to bring the prefabricated plant to off-shore gas wellswhile it would be impossible using the cumbersome processes of the priorart.

More specifically, the invention contemplates the preassembly of asimplified expansion type of liquefaction plant on a marine platformwhich is only a portion of the size of the conventional cascadeliquefaction plant disclosed in the above-cited patent, and which avoidsthe need for purification systems which employ molecular sieves andadsorbents that must be transported to a plant site and periodicallyregenerated, replaced and salvaged.

Accordingly, the present invention provides, in its preferred aspects, aprocess for producing a large supply of liquified natural gas in areasremotely located in the world where the source of power is limited ornon-existent and where the use of molecular sieves and adsorbents isuneconomic.

An object of the present invention is to provide a process which willproduce liquified natural gas employing moderate pressures (200-1000psia) and still maintain thermodynamic efficiency. Accordingly, thisefficiency is accomplished by controlling the condensing gas pressurewith relation to its analysis so that the temperature-enthalpy curve isessentially a straight line. This enhances the possibility ofrefrigeration by a cold gaseous stream eliminating many of the cryogenicliquid handling problems of other processes and which would be extremelydifficult and hazardous in floating platform construction as describedabove.

Another object of the present invention is to accomplish liquefaction byemploying a countercurrent heat exchanger-liquifier or subcooler wherebythe cooling or refrigerant stream is maintained in the form of a gasinstead of a boiling liquid refrigerant stream. This object can beaccomplished since it has been unexpectedly found that the nitrogencontent of the feed gas stabilizes in the refrigerant system and theactual refrigerant becomes an equilibrium mixture of methane andnitrogen gases. This analysis change assists greatly in providing therequired temperature driving force to promote the liquefaction of themain stream of natural gas which because of its heavier constituentscondenses at a higher temperature.

Still another object is to employ as the refrigerant material acombination of the flash gas obtained in the pressure letdown from theheat exchanger-liquifier and the cold expansion turbine effluent gasthereby causing the lower boiling point gases in the feed gas stream toreach an equilibrium in the expander refrigerant cycle. The refrigerantsystem is thus allowed to operate effectively at lower temperatures andpressures than processes heretofore employed. By being able to operateat lower temperatures and pressures, the equipment attending to theprocess of the present invention is far less expensive than processesheretofore employed.

It is another object of the present invention to form a bleed stream ofthe warmed effluent refrigerant gas from the heat exchanger-liquifierequivalent to the flow rate of the flash gas coming from the LNGgas-liquid separator. This bleed stream can be employed as fuel for agas turbine system which in turn furnishes part of the power required todrive the compressors in the refrigeration cycle.

An additional object of the present invention is to control thetemperature and pressure of the refrigerant gas so as continually tomaintain a gaseous state in the heat exchangers and expansion turbines.It has been unexpectedly found that if the temperature-enthalpy curve ofthe feed gas is sufficiently straight as controlled by its pressure itis not necessary to cool the refrigerant gas in the expansion engine tosuch an extent that some of the gas liquifies in order to liquify theincoming gas. Therefore, the present invention has a decided advantageover the prior art processes which partially liquify the gas in theexpansion turbines, thus eliminating the problem of erosion in the vanesof the turbine, which, in turn, becomes costly in the overall operatingexpense of the liquifaction plant.

In accordance with the present invention there is provided an integratedprocess for separating substantially all of the carbon dioxide and waterand most of the heavy hydrocarbons from raw natural gas feedstock, andthen liquifying the resulting purified natural gas mixture to form aproduct comprising nitrogen and hydrocarbons containing up to about 6carbon atoms. The raw natural gas mixture is purified by first producingthe crude mixture, under pressure, into an inlet separator vesselwherein condensed water and a hydrocarbon condensate are separated fromthe gas mixture as two substantially immiscible liquid layers. The waterlayer is drained from the bottom of the separator and discarded. Thecondensed hydrocarbon layer and the gaseous portion of the raw feed aretransferred from the separator to a cryogenic LNG fractionatordistilling all of the nitrogen, substantially all of the methane andessentially none of the carbon dioxide contained in the raw feed asoverhead. The bottoms from the LNG fractionator, which contain a smallamount of methane, aswell as all of the C hydrocarbons and essentiallyall of the carbon dioxide contained in the raw feed, is furtherfractionated to separate (a) the carbon dioxide, methane and part oftheethane at one end of the spectrum and (b) the C hydrocarbons at theother end of the spectrum from (c) the remainder of the hydrocarbons(predominantly ethane with but minor amounts of C and C hydrocarbons);the latter being combined with the portion of the LNG fractionatoroverhead not used for reflux. This combined stream is then liquified toform the LNG product. In a preferred embodiment, after removal of thecarbon dioxide fraction, the remaining C hydrocarbons are fractionatedin an LPG fractionator to separate a C -C fraction from a (3 gas liquidfraction, whereupon only the C -C fraction is combined with the overheadfrom the LNG fractionator, thereby forming a substantially purifiednatural gas mixture that is essentially free from carbon dioxide andcompletely devoid of C hydrocarbons. In any event, the carbon dioxidefraction split from the LNG fractionator bottoms may be used as fuel gasto supply part of the power requirements of the process equipment.

In the event of the temperature and pressure conditions of the raw gasmixture leaving the inlet separator are such that hydrates may formduring subsequent processing, a quantity of an inert dessicant, such asa solution of triethylene glycol, may be injected into the gas mixtureto reduce its dew point. This may be conveniently carried out, forexample, by injecting the triethylene glycol solution into the gasmixture as it leaves the inlet separator, whereafter the water-enrichedglycol solution is removed from the LNG fractionator bottoms by passingthe bottoms to a heavy hydrocarbon surge drum wherein the glycolsolution and hydrocarbons separate, the latter then being decanted andtransferred to the carbon dioxide fractionator.

The substantially purified natural gas comprising the LNG fractionatoroverhead and either the carbon dioxide free fraction from the carbondioxide fractionator or the C C fraction from the LPG fractionator isthen v cooled under elevated pressure by heat exchange with a combined,methanenitrogen refrigerant gas mixture obtained elsewhere in theprocess so as to effect liquefaction of a predominant portion of thepurified natural gas mixture while heating the combined refrigerant gasmixture. The pressure of the cooled natu ral gas is then substantiallyisenthalpically reduced, thereby further cooling it, forming (by flashevaporation) a minor gas fraction consisting essentially of methane andnitrogen and liquifying completely the balance of the natural gasmixture to form a major liquid fraction containing methane andsubstantially all of the C hydrocarbons present in the purified naturalgas feed. The minor, methane-nitrogen gas fraction is then separatedfrom the major liquid fraction and the major liquid fraction isrecovered as product.

The separated minor, methane-nitrogen gas fraction is combined with anisentropically expanded methanenitrogen refrigerant gas mixturecirculated in the process. This gas mixture can be produced elsewhere inthe process. This combination of methane-nitrogen gas constitutes thecombined refrigerant gas mixture mentioned previously and, as alsomentioned previously, this combined refrigerant gas mixture is employedto cool the purified natural gas mixture in the liquefaction heatexchange step. A major portion of the heated combined refrigerant gasmixture leaving the liquefaction heat exchange step is compressed toprovide a compressed, methane-nitrogen refrigerant gas mixture.

This gas mixture, after suitable precooling, is substantiallyisentropically expanded in one or more work recovery engines wherebymechanical energy is obtained, the gas mixture is further cooled, andthe isentropically expanded, methane-nitrogen refrigerant gas mixturementioned previously is obtained. The compressed methane-nitrogen gasmixture, isentropically expanded methane-nitrogen gas mixture and thecorresponding portion of the combined gas mixture constitute acirculating refrigerant system.

At least a major portion of the heated, combined refrigerant gas mixtureleaving the liquefaction heatexchange step and prior to compression isheat exchanged with the compressed, precooled methanenitrogenrefrigerant gas mixture whereby such combined gas mixture is furtherheated to a temperature suitable for entering the compressors.Concurrently, the compressed, methane-nitrogen refrigerant gas mixtureis cooled to a temperature level such that upon work expansion aspreviously described appropriate temperature levels and refrigerationcapacity are developed for liquefaction of the natural gas mixture.

A minor portion of the combined refrigerant gas mixture after leavingthe liquefaction heat exchange step is removed from the circulatingrefrigerant system and is employed as fuel gas to supply a portion ofthe power needed to drive the above-mentioned compressors. Although thisminor portion can be removed at any point in the circulating refrigerantsystem, it is preferred to remove it prior to compression of thecombined refrigerant gas mixture. Advantageously, this minor portion isremoved subsequent to the heat exchange of the combined refrigerant gasmixture with the compressed, precooled methane-nitrogen refrigerant gasmixture and prior to compression of the combined refrigerant gasmixture. The mechanical energy obtained from the work recovery enginesis also employed to provide at least part of the power needed tocompress the major portion of the combined gas mixture. The quantity ofthe minor portion removed from the refrigerant system is substantiallyequal in magnitude to the minor methane-nitrogen gas fraction formed bythe isenthalpic pressure reduction (flash evaporation) previouslydescribed.

The raw natural gas feedstock suitable for charging to the initialseparating vessel of this invention may comprise natural gas obtainedfrom a crude oil well (associated gas) or from a gas well(non-associated gas). However, various natural gas streams may not besuitable for immediate employment in the present invention due to thesource, both ultimate and immediate, from which they are obtained. Thus,for example, both associated and non-associated gas obtained directlyfrom the well head may be at an undesirably high temperature, e.g.substantially in excess of 100F. Further, associated gas or gas obtainedfrom certain storage facilities may be of inadequate pressure foremployment in the present process, eg only about lOO or 200 psia. Thus,certain natural gas streams, depending upon their source, may requirecertain pretreatment or preparation prior to employment in the presentinvention.

While there is no theoretical maximum to the pressure which can beemployed in the various fractionators and heat exchangers of the presentinvention, it would appear that there is little practical advantage tobe gained by employing pressures in excess of 1000 psia. This isparticularly true in connection with the heat exchanger or subcooleremployed in the liquefaction step unless the purified natural gasentering the subcooler contains a very high methane content Preferably amaximum pressure no greater than about 750 psia is employed, therebyavoiding the necessity of very high pressure vessels. Advantageously, amaximum pressure of about 650 psia is maintained. On the other hand, itis found .that a minimum pressure of about 350 psia is required for thisheat exchange liquefaction and preferably a pressure of about 600 psiais maintained. The pressure selected is such that a plot of thetemperature vs. enthalpy of the gas to be liquified approaches astraight line, i.e. not have a large enthalpy increase over a narrowtemperature range. Thus the pressure selected will depend on the gascomposition. Accordingly, therefore, a low pressure natural gas stream,such as an associated gas, can be compressed to achieve the desiredpressure range either prior to introduction into the initial separatorvessel or prior to introduction into the LNG fractionator, whereas anextremely high pressure natural gas stream, such as a nonassociated gas,can be reduced in pressure either prior to introduction to the initialseparator vessel or, preferably, after leaving the separator vessel butprior to introduction into the LNG fractionator. This reduction ofpressure can be accomplished by throttling, employment of aturbo-expander or any other expansion or pressure reducing means whethercapable of recovering mechanical energy or not.

In the event that the initial temperature of the natural gas is too highit must be cooled, such as, for example, by heat exchange or othertechniques well-known in the art. If the natural gas is at anundesirably high temperature and an undesirably high pressure it ispossible to reduce both the temperature and pressure of the gassimultaneously by the simple technique of adiabatic expansion. By use ofthese well-known techniques it is possible to limit the temperature ofthe natural gas charged to the LNG fractionator, for example, to amaximum of about 150F., preferably, however, the maximum temperaturewill not exceed about 125F. It is particularly advantageous to employtemperatures below about 100F. Selection of the exact processingtemperature can depend on the processing sequence selected for gaspurification as discussed below.

It will be understood, of course, that the selection of the particularoperating pressure as well as the temperature employed in the variousfractionators and in the liquefaction heat exchanger will be determinedto a great extent by the particular composition of the natural gas beingtreated as well as by the temperature of the natural gas being charged.In any event, however, it is necessary that the temperature and pressureconditions in the liquefaction heat exchanger be maintained such that atleast a predominant portion of the natural gas effluent therefrom is inthe liquid state, In this latter connection, the temperature andpressure conditions in the liquefaction heat exchanger are generallysuch that about 85 percent or more of such natural gas is in the liquidstate. Preferably at least about 90 percent, more preferably at leastabout 95 percent, is liquified. Usually the temperature of the coolednatural gas from the liquefaction heat exchange will be about -l75 toabout 225F. when employing pressures within the ranges indicated above.

After liquefaction heat exchange, the pressure of the natural gasisenthalpically reduced to a pressure just slightly above atmosphericpressure, generally falling in the range of from about 16 to about 35psia, and preferably in the range of from about 20 to about 30 psia.This isenthalphic reduction in pressure results in the flash evaporationof the minor gas fraction, liquefaction of the balance of the naturalgas and the overall reduction in temperature of both the minor gasfraction and the remaining major liquid fraction. The minor gas fractiongenerally comprises up to about 15 mol percent of the total liquifiednatural gas mixture and preferably comprises from about to about molpercent of the liquified natural gas mixture. Again the exact extent ofthe reduction in temperature effected by the isenthalpic pressurereduction is dependent primarily upon the magnitude of reduction inpressure effected and upon the composition of the natural gas mixture.Accordingly, the magnitude of the reduction in pressure must besufficient to reduce the temperature to a level sufficient to ensureliquefaction of a major portion of the particular composition of naturalgas at the lower pressure. Thus, by way of illustration, the isenthalpicreduction in pressure of some substantially completely liquified naturalgas mixtures from the range of about 600 psia down to about 20 psia willresult in a further reduction in temperature of the natural gas in therange of about 40 to about F. and the achievement of a temperaturesufficient to liquify completely the main portion of the stream at thelower pressure, e.g., 240 to 260F.

The minor gas fraction which has been flashed comprises about 95 molepercent (dry) methane and about 5 mole percent (dry) nitrogen, andcontains a substantial portion of the nitrogen initially contained inthe raw feedstock. The liquid fraction remaining after pressure let downcomprises about to percent of the methane and essentially all of the Chydrocarbons fed to the liquifier heat exchanger. The mole percent (dry)of nitrogen contained in the remaining liquid fraction is substantiallylower than the mole percent of nitrogen in the liquified natural gasprior to pressure let down.

After combining the above minor gas fraction with the isentropicallyexpanded methane-nitrogen refrigerant gas mixture, such combined gasmixture will be at a temperature lower than the temperature of theliquified natural gas from the liquefaction heat exchange, but will becompletely in the gaseous state.

As will be understood, the composition of the minor methane-nitrogen gasfraction determines the composition of the combined gas mixture, andwhile the quantity of the minor gas fraction is grossly smaller than thequantity of the purified natural gas mixture to be liquified, such minorgas fraction is allowed to build up and is recirculated within therefrigerant system such that during normal operation the flow rate ofthe combined gas mixture is from about 1.5 to about 4 times the flowrate of the purified natural gas mixture, expressed in moles per hour(dry). Preferably, the flow rate of the combined gas mixture is in therange from about 2 to about 3 times the flow rate of the purifiednatural gas mixture. The amount of refrigerant and combined gas employedis determined by the relative plots of temperature vs. enthalpy for thecombined gas and the natural gas being liquified so that these curves donot cross, thereby ensuring proper heat exchange in the gas liquifier.

After having achieved the desired flow rate of the combined gas mixture,it is essential to normal operation of the process for a quantity ofsuch combined gas mixture to be bled from the system after heat exchangewith the purified natural gas mixture. The quantity of the combined gasstream removed from the system is substantially equal in magnitude tothe minor methanenitrogen gas fraction obtained by flashing. Thisremoval is necessary to maintain the system in balance and to preventunwanted build-up within the system once normal operations have beenachieved. This removal of a minor portion of the heated combined gasmixture can be effected immediately subsequent to removal of thecombined gas mixture from the liquefaction heat exchanger.Alternatively, such bleed can be effected subsequent to heat exchangingthe combined gas mixture with the compressed methanenitrogen refrigerantgas mixture but prior to such compression. Removal of this later pointpermits heat exchanging a somewhat greater quantity of uncompressed gasagainst a somewhat smaller quantity of compressed gas and can favor heatbalances in certain design conditions. Again this minor gas fraction canbe removed after any portion of the compression step if for any reasonhigher pressure gas is desired. Although this bleeding can be effectedat substantially any place subsequent to removing the combined gasmixture from the liquefaction heat exchange and prior to combining theisentropically expanded methane-nitrogen refrigerant gas mixture withthe minor gas fraction obtained by flash vaporization, it is consideredto be advantageous to effect such bleed after heat exchange, but priorto the compression so as to avoid needlessly compressing a quantity ofgas which is to be removed from the refrigerant system in any event.

The compression of the combined methane-nitrogen refrigerant gas mixturecan be effected in either single or multistage compression.Advantageously, multistage compression is employed utilizing, forexample,

at least two stages and preferably at least three stages.

The multi-stage compression can be used together with interstagecooling. Such cooling can be effected via heat exchange so as to recoverheat values for use elsewhere or such cooling can be effected employingair or water as a cooling means simply for dissipating the heat from thesystem. Additionally, the compressed methane-nitrogen refrigerant gasmixture can be subjected to an after cooling prior to heat exchange withthe combined gas mixture. Generally, the heat exchange between thecombined gas mixture and the compressed methane-nitrogen refrigerant gasmixture as well as the interstage cooling is conducted. so as to providea gas temperature at compressor inlet, either single or multi-stage,above the cryogenic range, i.e. above about -50F. Preferably, thiscompressor inlet temperature is maintained above about 20F. andadvantageously above about 60F. On the other hand, however, the inlettemperature of the gas to be compressed must not be excessively high orit will place a needless burden upon the compression equipment.Accordingly, therefore, the maximum temperature employed is no greaterthan about 300F. and preferably is no greater than the temperatureproduced in a gas line in tropical climates, i.e. about 120F.Advantageously, however, the maximum temperature is no greater thanabout 90 to 100F. Again, when employing multi-stage compression, themaximum temperature to each compression stage can be controlled byinterstage cooling in the manner mentioned above.

The combined gas mixture is heat exchanged against the compressedmethane-nitrogen refrigerant gas mixture so as to reduce the temperatureof the compressed methane-nitrogen refrigerant gas mixture. This heatexchange is designed to control the temperature of the compressedrefrigerant gas to the work recovery engines so as to assist in controlof the final combined gas temperature to the natural gas liquefaction.This latter heat exchange is also designed to increase the temperatureof the combined gas mixture prior to compression since, from anequivalent viewpoint, the temperature of the combined gas from theliquefaction heat exchange is at a temperature which is undesirably low,e.g. -100 F. or lower, to be handled by typical commercial compressors.Additionally, the increase in the temperature of the gas due tocompression must be be offset, and if the temperature of the gas exitingfrom a compressor is too low, e.g. less than 32F, fresh water coolersare inoperable and air coolers are ineffective in many, if not most,climates to accomplish the desired temperature reduction.

The isentropic expansion of the compressed methane-nitrogen refrigerantgas mixture is conducted so as to cool the methane-nitrogen gas mixtureto as low a temperature as can be achieved but to a temperature aboutthe liquefaction point of such a mixture at the outlet pressure of therespective work recovery engines, the outlet pressure of the final workrecovery engine being maintained slightly above atmospheric pressure.Generally, this pressure is in the range from about 16 to about 35 psiaand preferably in the range from about 20 to about 30 psia. In anyevent, the outlet pressure of the work recovery engine is maintained atthe same level as the flash evaporated minor gas fraction. The expansiontemperature is controlled by the inlet temperature as previouslymentioned.

In the process of this invention the operating conditions of temperatureand pressure are maintained such that at all times the combined gasmixture, the compressed methane-nitrogen refrigerant gas mixture and theisentropically expanded methane-nitrogen refrigerant gas mixture aresubstantially completely in the gaseous state. To phrase it in anothermanner, there is little, if any, liquid in the refrigeration gas cycle,of this process. However, liquid can be generated if desired but whendesigned for liquid type of operation special expansion engine designsare necessary to prevent erosion and special liquid distributors arerequired in the heat exchange system.

The mechanical energy obtained from the respective work recovery enginesis employed to supply a portion of the horse-power required by thecompressionoperation. It will be understood, that since the samequantity of material is being compressed as is being expanded, and sinceneither the compressors nor work recovery engines, e.g., theturboexpanders, are 100 percent efficient, the mechanical energyavailable from the work recovery engines will provide only a portion ofthe horsepower required for compression. Accordingly, therefore, theremaining portion of the mechanical energy required for compression issupplied by utilizing the fuel gas streams bled at various stages fromthe system and by employing one or more external sources of energy.

In order to describe this invention in greater detail, reference is madeto the following specific example which will be described in connectionwith the accompanying two sheets of drawings designated FlGS. 1a and lband together illustrating an integral flow scheme. All compositions andpercentage in the example are given on a dry basis, unless specifiedotherwise.

Referring to FIG. 1a, 30,408 Mols per hour of raw natural non-associatedgas is delivered from a suitable source (not shown) through conduit 10to an inlet separator 12 at a pressure of about 2350 psia and a.temperature of about F. The composition in mol percent of the feedentering the inlet separator 12 is essentially as follows: 1.00%nitrogen, 2.00% carbon dioxide, 83.22% methane, 6.18% ethane, 2.76%propane, 1.50% butanes, and 3.34% C and heavier hydrocarbons. The rawfeed also includes a total of about 3430 pounds per hour of water. Inthe separator 12 about 3260 pounds per hour of condensed water and about2598 Mols per hour of a hydrocarbon condensate are separated from theremainder of the water saturated feed gas. The condensed water isremoved as bottoms via conduit 14.

The hydrocarbon condensate is transferred through conduit 16 to ademethanizer or LNG fractionator 18 at a rate of about 2598 Mols perhour. The composition in mol percent of the hydrocarbon condensate isessentially as follows: 0.35% nitrogen, 1.50% carbon dioxide, 49.37%methane, 7.75% ethane, 5.17% propane, 4.12% butanes, and 31.66% C andheavier hydrocarbons. The condensate also comprises about 49 pounds ofdissolved water.

A restriction orifice 20 is placed in the conduit 16 to control the flowrate of the condensate and to prevent rapid build up of pressure in thefractionator 18. The normal operating pressure of the fractionator 18 isabout 600 psia. The gas stream leaving the separator 12 through conduit22 contains about 121 pounds per hour or about 230 ppm of water. Thehydrate formation temperature of this gas is about 73F. Therefore, toprevent hydrate formation in either conduit 22 or in subsequentprocessing, about 3772 pounds per hour of a 62 percent triethyleneglycol solution is injected into conduit 22 near the outlet of theseparator 12 by conventional means (not shown). The glycol is diluted toabout 60 percent in picking up water from the gas in conduit 22, wherebythe dew point of the gas is reduced to below 50F. The freezing point ofthe glycol solution is well below the processing temperatures.

A minor portion of the high pressure gas in conduit 22, i.e., about 465Mols per hour (dry) is bled off through conduit 24 and is employed asfuel gas in a later stage of the process. The remaining major portion ofthe high pressure gas from conduit 22, i.e., about 27345 Mols per hour,is passed by means of conduit 26 (as shown in FIG. 1a) to a first stageexpansion turbine or turboexpander 28 (shown in FIG. lb), wherein thepressure of the gas is reduced from about 2350 psia to about 610 psiawith a corresponding reduction in temperature from about 85F. to about42F. The turboexpander 28 is designed to operate at about 80 per centadiabatic efficiency such that a considerable amount of power isdeveloped as the high pressure gas expands. As discussed below, thispower is utilized as a partial drive for the third stage refrigerationcompressor 84.

In passing through the turboexpander 28, about 8 mole percent of the gasin conduit 26 condenses. Accordingly, a mixed liquid and gas stream isremoved from turboexpander 28 (shown in FIG. 1b) by means of conduit 30and is passed via conduit 30 to LNG fractionator 18 (shown in FIG. Id)at a temperature of about 42F. and a pressure of about 610 psia. A smallquantity of triethylene glycol solution enters the fractionator 18 withthis mixed stream.

The LNG fractionator 18 is designed to separate between methane andcarbon dioxide with an overhead temperature of about 120F. and a bottomstemperature of about 120F. Typically, the fractionator l8 requires about19 actual trays at percent efficiency with a 0.55/1 L/D ratio. Thehydrocarbon condensate in conduit 16 is generally fed into thefractionator 18 at about the eighth tray, while the mixed liquid and gasstream in conduit 30 is fed into the fractionator at about thefourteenth tray.

The overhead of the fractionator 18 passes through conduit 32 where itis separated into two streams 34 and 36 for separate condensation. Thestream 34 is condensed in a conventional flat plate cryogenic heatexchanger 38 designed to condense a sufficient portion of the overheadvapors for gravity run back to the fractionator 18. The reflux isreturned at its bubble point.

The reflux exchanger 38 is cooled by part of the cold refrigerant gas asfrom a second stage expansion turbine 94 of the refrigeration cycledescribed hereinbelow. Flow of the cooling refrigerant to the exchanger38 is controlled by the top temperature of the fractionator 18.

The main portion of the fractionator overhead not needed for refluxflows through conduit 36 to the LNG condenser 40, also a flat platecryogenic exchanger. Prior to entering the exchanger 40, the overheadportion in conduit 36 is combined with an ethane, propane and butanefraction in line 124 and recovered from an LPG fractionator 114 (FIG.lb) in a later section of the process. The combined stream, whichbecomes the LNG product stream, then enters the heat exchanger 40through conduit 42 at a temperature of about 1 1 1 F. At this point, thecombined stream is a two phase system: about 25 percent being liquid.After passing through the exchanger 40, the temperature of the combinedLNG stream is reduced to about -1 16F. The pressure of the LNG stream atthis point is about 600 psia. The combined LNG stream then passesthrough the heat exchanger-subcooler 44 wherein it is reduced intemperature to about 212F. by a combined refrigerant gas formed in alater stage of the process and described below. Typically, the subcooler44 comprises a standard plate type cryogenic heat exchanger. Theeffluent from the subcooler 44 is let downlthrough conduit 46 to anexpansion valve 48 whereby the pressure is isenthalpically reduced fromabout 595 psia to about 25 psia. In passing through valve 48 about 7mole percent of the subcooled LNG stream flashes and the temperature ofthe mixture drops to about 246F. The pressure of about 25 psia isselected since it is just sufficient to drive the flash gas through theheat exchangers and to the regeneration gas compression system as willbe described in more detail hereinafter.

The cold and expanded mixture from the expansion valve 48 is conductedthrough conduit 50 to the LNG gas-liquid separating drum 52 which isnormally operated about half full of liquid.

The cold LNG liquid stream is passed from the separating drum 52 to astorage vessel (not shown) through conduit 54 at a flow rate of about25014 Mols per hour and at a pressure of about 25 psia. An analysis ofthe recovered liquid reveals the following composition in mol percent:0.70% nitrogen, 0.00% carbon dioxide, 90.61% methane, 4.80% ethane,3.20% propane, 0.69% butanes and 0.00% C hydrocarbons.

The residual cold flash gas leaves the LNG separating drum 52 throughconduit 56 at a flow rate of about 2196 Mols per hour and ata'temperatureof about 246F. The pressure of the LNG flash gas is about25 psia. The residual flash gas contains about 5.10% nitrogen and 94.90%methane, recorded as mol percent.

The residual flash gas is then combined at point 58 with 63215 Mols perhour of a work-expanded (isentropically expanded) and cooled refrigerantgas of line 61 from a third stage expansion turbine 60 so as to form thecombined refrigerant gas employed in subcooler 44. The combinedrefrigerant gas thus passes through conduit 62 and enters the subcooler44 at a flow rate of about 6541 1 Mols per hour and at a temperature andpressure of about -246F. and 25 psia, respectively. At a designatedtemperature and pressure, the combined refrigerant gas remains inessentially a gaseous state, so as to eliminate erosion of theturbo-expander blades and simplify distribution. Although it ispreferred that no liquid be present, a small quantity, i.e., 1 to 3percent, liquid in the form of a mist can be tolerated.

The combined refrigerant gas of conduit 62 emerges from subcooler 44through conduit 64 at -126F. and 21 psia, hereinafter referred to as therefrigerant gas, is passed to the first stage expander precooler 70 at aflow rate of about 63215 Mols per hour and at the same temperature andpressure as indicated hereinabove.

The low pressure refrigerant gas fraction in conduit 64 enters theexpander precooler 70 to cool the countercurrent flow of warm, highpressure, refrigerant gas from the compressor system. The warmed lowpressure, refrigerant gas then leaves precooler 70 through conduit 66 ata temperature of about 90F. and a pressure of about 16 psia. At point 68a minor bleed or regeneration-fuel fraction emerges through conduit 72at a flow rate approximately equal to the flow rate of the residualflash gas from the LNG gas-liquid separating drum 52, i.e., about 2196Mols per hour. As described more fully below, this latter fraction isemployed as fuel gas to supply part of the power required to operate thecompressor system utilized in the process. The remaining fraction ispassed by means of line 74 into a first stage compressor 76 wherein thepressure of the refrigerant gas is increased to about 61 psia. Thepartially compressed refrigerant gas is then forced through conduit 78to a second stage compressor 80, and then through conduit 82 to a thirdstage compressor 84 (shown in FIG. 1b). As illustrated in the drawing,intercoolers 85 and 86 are utilized to cool the refrigerant gas inconduits 78 and 82, respectively. After leaving the third stagecompressor 84 through conduit 88, the compressed refrigerant gas is at apressure of about 572 psia. The compressed refrigerant gas is thenpassed through after cooler 90 wherein the temperature is reduced toabout 95F., the pressure dropping slightly to about 562 psia. Thecompressed and cooled refrigerant gas is then passed to the expandedprecooler 70 (shown in FIG. 1a) to be precooled by the incoming lowpressure refrigerant gas from conduit 64. The precooled, compressedrefrigerant gas from the precooler 70 then passes through conduit 92 toa second stage enpansion turbine 94. The flow rate of the gas enteringthe second stage expansion turbine is about 63215 Mols per hour at atemperature of 8 1F. and a pressure of 560 psia. The second stagerefrigerant expander gas leaves the second stage expansion turbine 94 atabout 190 psia and 170F. and is passed through conduit 96 to thecryogenic heat exchangers 38 and 40 where it is utilizied as therefrigerant. After emerging from the heat exchangers 38 and 40, thewarmed refrigerant gas is at a temperature of 126F. and a pressure of185 psia. The refrigerant gas is then passed through conduit 98 to athird stage expansion turbine wherein the pressure is further reduced toabout 25 psia with a corresponding decrease in temperature to about246F. As pointed out above, this low pressure, low temperaturerefrigerant gas is passed via conduit 61 and combined at point 58 withthe LNG flash gas in conduit 56 to form the combined refrigerant gaswhich is passed via conduit 62 through the subcooler 44 to reduce thetemperature and thereby completely liquify the LNG product streamcarried in conduit 42.

The liquid bottoms from the fractionator 18 contains substantially allof the carbon dioxide and higher boiling materials. This bottoms leavesthe fractionator 18 at the reboiler temperature of 140F. and flows underthe fractionator pressure through conduit 100 to point 102 where theglycol solution is removed therefrom by conventional means (not shown).

The composition in me] percent of the glycol-free bottoms stream is asfollows: 11.89% carbon dioxide, 5.34% methane, 37.14% ethane, 16.57%propane, 9.01% butanes, and 20.05% C hydrocarbons. This stream thenpasses at a rate of about 5003 Mols per hour through conduit 104 to theCO fractionator 106 (shown in FIG. 1b). The fractionator 106 operates at550 psia distilling overhead at 20F. an azeot'rope of approximatelyequal molar quantities of ethane and carbon dioxide. 1n addition,'theoverhead contains the methane left in the bottoms from the LNGfractionator 18. The bottoms of the CO fractionator 106 is substantiallyfree from carbon dioxide. The fractionator 106 is reboiled at about195F., while overhead reflux is supplied by a refrigerant such aspropane at about 15F. in an internal cooler 108. The use of the internalcooler 108 is desirable since it saves equipment and space. Thefractionator 1'06 typically comprises about 35 trays and operates at anL/D ratio of about 2/1. The overhead from fractionator 106 is passedthrough conduit 110 and may be employed as fuel gas. in a preferredembodiment, the pressure of the overhead is reduced, e.g., to about 250psia with a corresponding reduction in temperature to about 14F. in thismanner, the cooled overhead can be employed to supply part of thecooling capacity required in throughout the process being used as fuelgas.

The composition in mol percent of the liquid bottoms from the COfractionator 106 is approximately as follows: 35.48% ethane, 23.45%propane, 12.73% butanes, and 28.34% C5 hydrocarbons. This bottoms streamis transferred at a rate of about 3545 Mols per hour and a pressure ofabout 555 through conduit 112 to the LPG fractionator 1 14.

The LPG fractionator 114 operates at about 375 psia and fractionates anoverhead containing ethane, propane and as much butane as can betolerated to the solubility limit in LNG. To increase the quantity ofbutane in the overhead, a rough cut may be made between isoandnormal-butane as the isois more soluble in LNG. As in the case of the COfractionator 106, the overhead of the LPG fractionator 114 is condensedby an internal cooler 116 to save equipment and space at the plant site.Typically, water is employed as the refrigerant in cooler 116. Thefractionator 114 is designed to operate with about 25 trays and a L/Dratio of about Ill.

The gas liquids bottoms from the fractionator 116 is withdrawn at atemperature of about 470F. and is transferred to storage through conduit118 at a rate of about 1276 Mols per hour. The composition in molpercent of this gas liquids product stream is approximately as follows:0.10% propane, 20.47% butanes, 16.42% pentanes, 14.52% C and 48.49% Chydrocarbons.

The overhead vapors from the LPG fractionator 116 pass through conduit120 to a compressor-condenser unit 122 where they are compressed toabout 635 psia and then condensed to a liquid by cooling to about lOF.The condensed and cooled C fraction is then passed through conduit 124for admixture with the major portion of the overhead from the LNGfractionator 18 (shown in FIG. 1a).

As discussed above, it is this mixed stream which passes through conduit42 and ultimately forms the LNG product stream.

As a result of the expansion in the turboexpanders 28, 60 and 94, workis produced which furnishes a per tion of the power necessary to drivethe compressors 84, 80 and 76. The power from the turboexpanders istransferred to the compressors by connecting the respective compressorsand expanders with common shafts 126. Other suitable mechanical meanscan also be used, such as gears, torque converters, etc. The additionalpower required to drive the various compressors is provided by separateconventional gas turbines 128 connected to each of the compressors 76,80 and 84 by common shafts 130. Typically, the bleed gas or fuel gasremoved from the process in conduits 24, 72 or 110 is employed to drivethe gas turbines 138. Steam turbines can also be used as an alternatedrive. While each of the gas turbines 128 is shown as being operativelyconnected with a compressor by means of a common shaft 130, it will beunderstood that any other suitable mechanical linkage, such as, a gearchain, belt and pulley system, etc, can also be employed.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art from the foregoing discussion andaccompanying drawing without departing from the scope and spirit of thisinvention and this invention is not to be limited unduly to that setforth herein for illustrative purposes.

What is claimed is:

l. A process for liquifying a natural gas mixture comprising nitrogen,carbon dioxide, water and lower boiling hydrocarbons so as to produce asubstantially carbon dioxideand water-free liquified natural gas, whichprocess comprises:

1. Condensing the natural gas mixture under elevated pressure to form(a) a liquid water phase, (b) a heavy hydrocarbon liquid phasecontaining nitrogen and carbon dioxide and (c) a light hydrocarbon vaporphase containing nitrogen and carbon dioxide;

2. Removing the liquid water phase from the process;

3. Substantially isentropically expanding the light hydrocarbon vaporphase in a work recovery engine to obtain mechanical energy and to coolthe light hydrocarbon vapor phase;

4. Fractionating the heavy hydrocarbon liquid phase from Step (1) andthe cooled light hydrocarbon vapor phase from Step (3) to form (a) amethanenitrogen fraction substantially free of carbon dioxide andhydrocarbons heavier than methane and (b) a carbon dioxide-hydrocarbonfraction substantially free of nitrogen and containing C and heavierhydrocarbons;

5. Fractionating the carbon dioxide-hydrocarbon fraction from Step (4)into (a) a substantially carbon dioxide fraction, (b) a C and heavierhydrocarbon fraction and, (c) C to C hydrocarbon fraction and removingthe carbon dioxide and C and heavier hydrocarbon fractions from theprocess;

6. Compressing and cooling the C to C hydrocarbon fraction from Step (5)to form a low temperature liquid stream;

7. Combining the low temperature liquid stream of Step (6) with at leasta portion of the methanenitrogen fraction from Step (4) so as to form anitrogen-methane and C to C hydrocarbon mixture;

8. Cooling the nitrogen-methane and C to C 9. Substantiallyisenthalpically reducing the pressure of the cooled predominantlyliquid, liquid-vapor nitrogenmethane and C to C hydrocarbon mixture soas further to cool it and to form a minor gas fraction consistingessentially of a mixture of methane and nitrogen while liquifyingcompletely the balance of the cooled stream to form a major liquidfraction containing methane and substantially all the hydrocarbonsheavier than methane through C 10. Separating the minor,methane-nitrogen gas fraction from the major liquid fraction andrecovering the major liquid fraction as liquified natural gas product;

l l. Combining the minor, methane-nitrogen gas fraction with anisentropically expanded, refrigerant gas mixture produced subsequentlyand consisting essentially of methane and nitrogen to form the combinedgas mixture;

12. Employing the combined gas mixture formed in Step (ll) at leastpartially to cool the nitrogen methane and C to C hydrocarbon mixture inStep l3. Compressing a major portion of the heated, combined gas mixturefrom Step (8) to provide a compressed refrigerant gas mixture;

14. Heat exchanging at least the major portion of the combined gasmixture prior to the compression of Step (13) with the compressedrefrigerant gas mixture whereby the at least major portion of theheated, combined gas mixture is further heated to a temperature fromabout -50 to about 300F. and the compressed refrigerant gas mixture iscooled;

15. Additionally cooling the compressed refrigerant gas to a temperaturesuch that in expansion in a work recovery engine the temperature andenthalpy are sufficiently low to provide the refrigeration required forStep (8);

' l6. Substantially isentropically expanding the additionally cooled,compressed refrigerant gas mixture in a work recovery engine to obtainmechanical energy and to further cool the refrigerant gas, whereby theisentropically expanded, refrigerant gas mixture is formed;

17. Removing from the process system subsequent to Step (8) a minorportion of the combined gas mixture substantially equal in magnitude tothe minor methane-nitrogen gas fraction formed in Step (9); and

18. Employing the mechanical energy obtained from the work recoveryengines of Steps (3) and l6) to compress, at least partly, the majorportion of the combined refrigerant gas mixture in Step l3 2. Theprocess of claim 1 wherein the nitrogenmethane and C to C hydrocarbonmixture in Step (8) is at a pressure in the range of from about 350 toabout 750 psia.

3. The process of claim 1 wherein the nitrogenmethane and C to Chydrocarbon mixture in Step (8) is maintained at a pressure relative toits analysis such that the plot of the temperature vs. enthalpy of themix-- ture approaches as straight a line as is reasonably possible.

' 4. The process of claim 1 wherein the flow rateof the combinedmethane-nitrogen gas mixture in Step (8) is from about 1.5 to about 4times the flow rate of the nitrogen-methane and C to C hydrocarbonmixture in Step (8), expressed as Mols per hour.

5. The process of claim 3 wherein the flow rate of the combinedmethane-nitrogen gas in Step (8) is maintained at such a rate that thetemperature vs. enthalpy curve for the combined gas as closely aspossible parallels the temperature vs. enthalpy curve of thenitrogenmethane and C to C hydrocarbon mixture; whereby more effectiveheat exchange between the combined gas and the nitrogen-methane and C toC hydrocarbon mixture is effected.

6. The process of claim 1 wherein the compressed, methane nitrogenrefrigerant gas mixture from Step 13) is at a pressure from about 400 toabout 600 psia.

7. The process of claim 1 wherein the compression of Step (13) iseffected in a plurality of stages and the gas is cooled interstage.

8. The process of claim 1 wherein the minor portion of the combined gasmixture removed in Step (17) is charged to a separate work recoveryengine whereby mechanical energy is obtained, and wherein the mechanicalenergy as obtained is employed at least to compress partly the majorportion of the combined gas I 1. Condensing the natural gas mixtureunder elevated pressure to form (a) a liquid water phase, (b) a heavyhydrocarbon liquid phase containing nitrogen and carbon dioxide and (c)a light hydrocarbon vapor phase containing nitrogen and carbon dioxide;

. Removing the liquid water phase from the process;

. Substantially isentropically expanding the light hydrocarbon vaporphase in a work recovery engine to obtain mechanical energy and to coolthe light hydrocarbon vapor phase;

4. Fractionating the heavy hydrocarbon liquid phase from Step (1) andthe cooled light hydrocarbon vapor phase from Step (3) to form (a) amethanenitrogen fraction substantially free of carbon dioxide andhydrocarbons heavier than methane and (b) a carbon dioxide-hydrocarbonfraction substantially free of nitrogen and containing C and heavierhydrocarbons;

5. Separating the methane-nitrogen fraction into a first portion and asecond portion;

6. Heat exchanging the first portion with a first isentropicallyexpanded methane-nitrogen refrigerant gas obtained subsequently so as toremove heat from the first portion and to heat the first isentropicallyexpanded methane-nitrogen refrigerant gas;

7. Returning the heat exchanged first portion to the fractionation ofStep (4) as reflux;

8. Fractionating the carbon dioxide-hydrocarbon fraction from Step (4)to separate carbon dioxide from the C and heavier hydrocarbons andremoving the carbon dioxide from the process;

9. Fractionating the C and heavier hydrocarbons from Step (8) to form aC C, hydrocarbon fraction and a C and heavier hydrocarbon fraction andremoving the C and heavier hydrocarbon fraction from the process;

l0. Compressing and cooling the C -C hydrocarbon fraction from Step (9)to form a low temperature liquid stream;

1 l. Combining the low temperature liquid stream of Step l0) with thesecond portion of the methanenitrogen fraction from Step (4) so as toform a nitrogen-methane and C -C hydrocarbon mixture;

l2. Cooling the nitrogen-methane and C -C hydrocarbon mixture underelevated pressure by heat exchange with a first isentropically expandedmethane-nitrogen refrigerant gas obtained subsequently, thereby heatingthe first isentropically expanded refrigerant gas;

13. Cooling further the nitrogen-methane and C -C hydrocarbon mixtureunder elevated pressure by heat exchange with a combined gas mixtureconsisting essentially of methane and nitrogen produced subsequentlywhereby the combined gas mixture is heated and a predominant portion ofthe nitrogenmethane and C C. hydrocarbon mixture is liquified so as toform a predominantly liquid, liquid-vapor mixture;

14. Substantially isenthalpically reducing the pressure of the cooledpredominantly liquid, liquidvapor nitrogen-methane and C -C hydrocarbonmixture so as further to cool it and to form a minor gas fractionconsisting essentially of a mixture of LAN methane and nitrogen whileliquifying completely the balance of the cooled stream to form a majorliquid fraction containing methane and the C -C hydrocarbons;

l5. Separating the minor, methane-nitrogen gas fraction from the majorliquid fraction and recovering the major liquid fraction as liquifiednatural gas product;

16. Combining the minor, methane-nitrogen gas fraction with a secondisentropically expanded, refrigerant gas mixture produced subsequentlyand consisting essentially of methane and nitrogen to form the combinedgas mixture; 7

l7. Employing the combined gas mixture formed in Step 16) to cool thenitrogen-methane and C -C hydrocarbon mixture in Step l3 l8. Compressingat least a major portion of the heated, combined gas mixture from Step(13) to provide a compressed refrigerant gas mixture;

19. Heat exchanging at least a major portion of the combined gas mixtureprior to the compression of Step (18) with the compressed refrigerantgas mixture whereby the at least major portion of the heated, combinedgas mixture is further heated to a temperature from about 50 to about300F. and the compressed refrigerant gas mixture is cooled;

20. Additionally cooling the compressed refrigerant gas to a temperaturesuch that in expansion in a work recovery engine the temperature andenthalpy are sufficiently low to provide the refrigeration required forSteps l2) and l3 2l. Substantially isentropically expanding theadditionally cooled, compressed refrigerant gas mixture in a workrecovery engine to obtain mechanical energy and to further cool therefrigerant gas, whereby the first isentropically expanded,methane-nitrogen refrigerant gas is formed;

22. Substantially isentropically expanding the heated firstisentropically expanded refrigerant gas from Step (12) in a workrecovery engine whereby (a) mechanical energy is obtained, (b) theheated refrigerant gas is cooled, and (c) the second isentropicallyexpanded methane-nitrogen refrigerant gas is formed; r

23. Removing from the process system subsequent to Step (13) a minorportion of the combined gas mixture substantially equal in magnitude tothe minor methane-nitrogen gas fraction formed in Step l4); and

24. Employing the mechanical energy obtained from the work recoveryengine of Steps (3), (21) and (22) at least to compress partly the majorportion of the combined refrigerant gas mixture in Step (l8).

10. The process of claim 9 wherein the nitrogenmethane and C Chydrocarbon mixture in Step (13) is at a pressure in the range fromabout 350 to about 750 psia.

11. The process of claim 9 wherein the nitrogenmethane and C Chydrocarbon mixture in Step (13) is maintained at a pressure relative toits analysis such that the plot of the temperature vs. enthalpy of themixble.

12. The process of claim 9 wherein the flow rate of the combinedmethane-nitrogen gas mixture in Step 13) is from about 1.5 to about fourtimes the flow rate of the nitrogen-methane and C -C hydrocarbon mixturein Step l3 expressed as moles per hour.

13. The process of claim 12 wherein the flow rate of the combinedmethane-nitrogen gas is maintained at such a rate that the temperaturevs. enthalpy curve for the combined gas as closely as possibleparallels-the temperature vs. enthalpy curve of the nitrogenmethane andC -C hydrocarbon gas mixture, whereby more effective heat exchangebetween the combined gas and the nitrogen-methane and C -C hydrocarbongas mixture is effected.

14. The process of claim 9 wherein the compressed, methane-nitrogenrefrigerant gas mixture from Step 18) is at a pressure from about 400 toabout 600 psia.

15. The process of claim 9 wherein the compression of Step (18) iseffected in a plurality of stages and the gas is cooled interstage.

16. A continuous process for the liquefaction of natural gas whichcomprises the steps of:

Condensing the natural gas mixture under elevated pressure to form (a) aliquid water phase, (b) a heavy hydrocarbon liquid phase containingnitrogen and carbon dioxide and (c) a light hydrocarbon vapor phasecontaining nitrogen and carbon dioxide;

removing the liquid water phase from the process;

adjusting the pressure of the heavy hydrocarbon liquid phase and thelight hydrocarbon vapor phase to a range of from about 350 to about 750psia;

fractionating the pressure adjusted heavy and light hydrocarbon phasesto form (a) a methanenitrogen fraction substantially free of carbondioxide and hydrocarbons heavier than methane and (b) a carbondioxide-hydrocarbon fraction substantially free of nitrogen andcontaining C and heavier hydrocarbons;

fractionating further the carbon dioxide-hydrocarbon fraction to form aC C hydrocarbon fraction and a C and heavier hydrocarbon fraction andremoving the C and heavier hydrocarbon fraction from the process;

combining the C C hydrocarbon fraction with the carbon dioxide freemethane-nitrogen fraction to form a substantially purified natural gasmixture;

cooling and substantially completely liquifying the purified natural gasmixture in a countercurrent heat exchanger-liquifier;

flash evaporating a minor gas fraction from the cooled effluent from theheat exchanger-liquifier by substantially isenthalpically reducing thepressure thereof, said flash evaporation causing the temperature of boththe minor gas fraction and the remaining major liquid fraction todecrease;

collecting the major liquid fraction as product and returning theflashed fraction to a point where it is combined with an isentropicallyexpanded refrigeration gas fraction produced subsequently in theprocess;

passing said combined gases countercurrently to the purified natural gasmixture in said heat exchanger-liquifier at a flow rate of two to fourtimes that of the purified natural gas stream;

separating the warmed effluent from said heat exchanger-liquifier into amajor refrigeration gas fraction and a minor fuel gas fraction;

passing the refrigeration gas fraction through a counter-current heatexchanger precooler to a compressor system to be compressed to apressure in the range of about 400 to 600 psia;

passing the pressurized gas from said compressor system through saidheat exchanger precooler to cool the refrigeration gas;

isentropically expanding the cooled effluent from said heat exchangerprecooler with an expansion engine at a temperature and pressuresufficient to maintain the gas in the gaseous state in said expansionturbine to form the isentropically expanded refrigeration gas, wherebymechanical energy is obtained and the pressure of the refrigeration gasfraction is reduced, thereby further cooling such fraction;

combining the further cooled, reduced pressure, isentropically expandedrefrigeration gas effluent from said expansion engine with said flashgas to a point prior to entry into said heat exchanger-liquifier; and

utilizing (a) the fuel gas fraction to furnish fuel for a gas driventurbine which provides a portion of the mechanical energy necessary todrive said compressor system and (b) the mechanical energy obtained fromthe isentropic expansion to provide an additional portion of themechanical energy necessary to drive the said compressor system.

17. In an integrated apparatus for purifying and liquifying natural gas,the combination comprising:

gas-liquid separating means for separating condensed water and a heavyhydrocarbon condensate from a pressurized natural gas mixture comprisingnitrogen, carbon dioxide, water and lower boiling hydrocarbons;

means for adjusting the pressure of the heavy hydrocarbon condensate andthe water and heavy hydrocarbon condensate-free remainder of the naturalgas mixture to about 350 to about 750 psia;

means for fractionating the combined, pressure adjusted hydrocarboncondensate and water-free natural gas mixture to form a methane-nitrogenfraction substantially free of carbon dioxide and a carbondioxide-hydrocarbon fraction substantially free of nitrogen andcontaining predominantly C hydrocarbons;

means for fractionating the carbon dioxidehydrocarbon fraction to form acarbon dioxide fraction and a C hydrocarbon fraction, said carbondioxide fraction including all of the methane present in the carbondioxide-hydrocarbon and a portion of the C hydrocarbons containedtherein;

means for combining the C hydrocarbon fraction with the methane-nitrogenfraction, thus forming a substantially water-free and carbondioxide-free natural gas mixture;

a heat exchanger-liquifier for cooling and liquifying the water-free andcarbon dioxide-free natural gas by countercurrent heat exchange with acombined flash gas and work expanded refrigerant gas from an expansionengine;

means for isenthalpically expanding and partially flashing the naturalgas effluent from said heat exchan er-liquitier; gas-liqui separatingmeans for separating said expanded effluent into a major liquid fractionand a minor flash gas fraction;

means for recovering the major liquid fraction as product;

means for passing the flash gas to the hea exchanger-liquifier;

compression means for compressing the combined flash gas and workexpanded refrigerant gas from the heat exchanger-liquifier;

a heat exchanger precooler for precooling the gas coming from thecompression means by countercurrent heat exchange with the combinedflash gas and work expanded gas from said heat exchangerliquifier;

an expansion engine for work expanding said precooled gas, said enginefurther cooling the precooled gas and reducing the pressure thereofwhile generating mechanical energy;

means for feeding the work expanded and cooled gas from said expansionengine to said heat exchangerliquifier to join said flash gas; and

means for bleeding from the apparatus a minor portion of the combinedflash gas and work expanded gas prior to compression thereof, saidexpansion engine and said compression means being operativelycooperative whereby the mechanical energy obtained from said enginesupplies at least a portion of the work required'to operate saidcompression means.

2. Removing the liquid water phase from the process;
 2. Removing theliquid water phase from the process;
 2. The process of claim 1 whereinthe nitrogen-methane and C2 to C5 hydrocarbon mixture in Step (8) is ata pressure in the range of from about 350 to about 750 psia.
 3. Theprocess of claim 1 wherein the nitrogen-methane and C2 to C5 hydrocarbonmixture in Step (8) is maintained at a pressure relative to its analysissuch that the plot of the temperature vs. enthalpy of the mixtureapproaches as straight a line as is reasonably possible. 3.Substantially isentropically expanding the light hydrocarbon vapor phasein a work recovery engine to obtain mechanical energy and to cool thelight hydrocarbon vapor phase;
 3. Substantially isentropically expandingthe light hydrocarbon vapor phase in a work recovery engine to obtainmechanical energy and to cool the light hydrocarbon vapor phase; 4.Fractionating the heavy hydrocarbon liquid phase from Step (1) and thecooled light hydrocarbon vapor phase from Step (3) to form (a) amethane-nitrogen fraction substantially free of carbon dioxide andhydrocarbons heavier than methane and (b) a carbon dioxide-hydrocarbonfraction substantially free of nitrogen and containing C2 and heavierhydrocarbons;
 4. Fractionating the heavy hydrocarbon liquid phase fromStep (1) and the cooled light hydrocarbon vapor phase from Step (3) toform (a) a meThane-nitrogen fraction substantially free of carbondioxide and hydrocarbons heavier than methane and (b) a carbondioxide-hydrocarbon fraction substantially free of nitrogen andcontaining C2 and heavier hydrocarbons;
 4. The process of claim 1wherein the flow rate of the combined methane-nitrogen gas mixture inStep (8) is from about 1.5 to about 4 times the flow rate of thenitrogen-methane and C2 to C5 hydrocarbon mixture in Step (8), expressedas Mols per hour.
 5. The process of claim 3 wherein the flow rate of thecombined methane-nitrogen gas in Step (8) is maintained at such a ratethat the temperature vs. enthalpy curve for the combined gas as closelyas possible parallels the temperature vs. enthalpy curve of thenitrogen-methane and C2 to C5 hydrocarbon mixture; whereby moreeffective heat exchange between the combined gas and thenitrogen-methane and C2 to C5 hydrocarbon mixture is effected. 5.Separating the methane-nitrogen fraction into a first portion and asecond portion;
 5. Fractionating the carbon dioxide-hydrocarbon fractionfrom Step (4) into (a) a substantially carbon dioxide fraction, (b) a C5and heavier hydrocarbon fraction and, (c) C2 to C5 hydrocarbon fractionand removing the carbon dioxide and C5 and heavier hydrocarbon fractionsfrom the process;
 6. Compressing and cooling the C2 to C5 hydrocarbonfraction from Step (5) to form a low temperature liquid stream;
 6. Heatexchanging the first portion with a first isentropically expandedmethane-nitrogen refrigerant gas obtained subsequently so as to removeheat from the first portion and to heat the first isentropicallyexpanded methane-nitrogen refrigerant gas;
 6. The process of claim 1wherein the compressed, methane nitrogen refrigerant gas mixture fromStep (13) is at a pressure from about 400 to about 600 psia.
 7. Theprocess of claim 1 wherein the compression of Step (13) is effected in aplurality of stages and the gas is cooled interstage.
 7. Combining thelow temperature liquid stream of Step (6) with at least a portion of themethane-nitrogen fraction from Step (4) so as to form a nitrogen-methaneand C2 to C5 hydrocarbon mixture;
 7. Returning the heat exchanged firstportion to the fractionation of Step (4) as reflux;
 8. Fractionating thecarbon dioxide-hydrocarbon fraction from Step (4) to separate carbondioxide from the C2 and heavier hydrocarbons and removing the carbondioxide from the process;
 8. The process of claim 1 wherein the minorportion of the combined gas mixture removed in Step (17) is charged to aseparate work recovery engine whereby mechanical energy is obtained, andwherein the mechanical energy as obtained is employed at least tocompress partly the major portion of the combined gas mixture. 8.Cooling the nitrogen-methane and C2 to C5 hydrocarbon mixture underelevated pressure by heat exchange with a combined gas mixtureconsisting essentially of methane and nitrogen produced subsequentlywhereby the combined gas mixture is heated and a predominant portion ofthe nitrogen-methane and C2 to C5 hydrocarbon mixture is liquified so asto form a predominantly liquid, liquid-vapor mixture;
 9. A process forliquifying a natural gas mixture comprising nitrogen, carbon dioxide,water and lower boiling hydrocarbons so as to produce a substantiallycarbon dioxide- and water-free liquid natural gas, which processcomprises:
 9. Fractionating the C2 and heavier hydrocarbons from Step(8) to form a C2-C4 hydrocarbon fraction and a C4 and heavierhydrocarbon fraction and removing the C4 and heavier hydrocarbonfraction from the process;
 9. Substantially isenthalpically reducing thepressure of the cooled predominantly liquid, liquid-vapornitrogenmethane and C2 to C5 hydrocarbon mixture so as further to coolit and to form a minor gas fraction consisting essentially of a mixtureof methane and nitrogen while liquifying completely the balance of thecooled stream to form a major liquid fraction containing methane andsubstantially all the hydrocarbons heavier than methane through C5; 10.Separating the minor, methane-nitrogen gas fraction from the majorliquid fraction and recovering the major liquid fraction as liquifiednatural gas product;
 10. Compressing and cooling the C2-C4 hydrocarbonfraction from Step (9) to form a low temperature liquid stream;
 10. Theprocess of claim 9 wherein the nitrogen-methane and C2-C4 hydrocarbonmixture in Step (13) is at a pressure in the range from about 350 toabout 750 psia.
 11. The process of claim 9 wherein the nitrogen-methaneand C2-C4 hydrocarbon mixture in Step (13) is maintained at a pressurerelative to its analysis such that the plot of the temperature vs.enthalpy of the mixture approaches as straight a line as is reasonablypossible.
 11. Combining the low temperature liquid stream of Step (10)with the second portion of the methane-nitrogen fraction from Step (4)so as to form a nitrogen-methane and C2-C4 hydrocarbon mixture; 11.Combining the minor, methane-nitrogen gas fraction with anisentropically expanded, refrigerant gas mixture produced subsequentlyand consisting essentially of methane and nitrogen to form the combinedgas mixture;
 12. Employing the combined gas mixture formed in Step (11)at least partially to cool the nitrogen-methane and C2 to C5 hydrocarbonmixture in Step (8);
 12. Cooling the nitrogen-methane and C2-C4hydrocarbon mixture under elevated pressure by heat exchange with afirst isentropically expanded methane-nitrogen refrigerant gas obtainedsubsequently, thereby heating the first isentropically expandedrefrigerant gas;
 12. The process of claim 9 wherein the flow rate of thecombined methane-nitrogen gas mixture in Step (13) is from about 1.5 toabout four times the flow rate of the nitrogen-methane and C2-C4hydrocarbon mixture in Step (13), expressed as moles per hour.
 13. Theprocess of claim 12 wherein the flow rate of the combinedmethane-nitrogen gas is maintained at such a rate that the temperaturevs. enthalpy curve for the combined gas as closely as possible parallelsthe temperature vs. enthalpy curve of the nitrogen-methane and C2-C4hydrocarbon gas mixture, whereby more effective heat exchange betweenthe combined gas and the nitrogen-methane and C2-C4 hydrocarbon gasmixture is effected.
 13. Cooling further the nitrogen-methane and C2-C4hydrocarbon mixture under elevated pressure by heat exchange with acombined gas mixture consisting essentially of methane and nitrogenproduced subsequently whereby the combined gas mixture is heated and apredominant portion of the nitrogenmethane and C2-C4 hydrocarbon mixtureis liquified so as to form a predominantly liquid, liquid-vapor mixture;13. Compressing a major portion of the heated, combined gas mixture fromStep (8) to provide a compressed refrigerant gas mixture;
 14. Heatexchanging at least the major portion of the combined gas mixture priorto the compression of Step (13) with the compressed refrigerant gasmixture whereby the at least major portion of the heated, combiNed gasmixture is further heated to a temperature from about -50* to about300*F. and the compressed refrigerant gas mixture is cooled; 14.Substantially isenthalpically reducing the pressure of the cooledpredominantly liquid, liquid-vapor nitrogen-methane and C2-C4hydrocarbon mixture so as further to cool it and to form a minor gasfraction consisting essentially of a mixture of methane and nitrogenwhile liquifying completely the balance of the cooled stream to form amajor liquid fraction containing methane and the C2-C4 hydrocarbons; 14.The process of claim 9 wherein the compressed, methane-nitrogenrefrigerant gas mixture from Step (18) is at a pressure from about 400to about 600 psia.
 15. The process of claim 9 wherein the compression ofStep (18) is effected in a plurality of stages and the gas is cooledinterstage.
 15. Separating the minor, methane-nitrogen gas fraction fromthe major liquid fraction and recovering the major liquid fraction asliquified natural gas product;
 15. Additionally cooling the compressedrefrigerant gas to a temperature such that in expansion in a workrecovery engine the temperature and enthalpy are sufficiently low toprovide the refrigeration required for Step (8);
 16. Substantiallyisentropically expanding the additionally cooled, compressed refrigerantgas mixture in a work recovery engine to obtain mechanical energy and tofurther cool the refrigerant gas, whereby the isentropically expanded,refrigerant gas mixture is formed;
 16. Combining the minor,methane-nitrogen gas fraction with a second isentropically expanded,refrigerant gas mixture produced subsequently and consisting essentiallyof methane and nitrogen to form the combined gas mixture;
 16. Acontinuous process for the liquefaction of natural gas which comprisesthe steps of: Condensing the natural gas mixture under elevated pressureto form (a) a liquid water phase, (b) a heavy hydrocarbon liquid phasecontaining nitrogen and carbon dioxide and (c) a light hydrocarbon vaporphase containing nitrogen and carbon dioxide; removing the liquid waterphase from the process; adjusting the pressure of the heavy hydrocarbonliquid phase and the light hydrocarbon vapor phase to a range of fromabout 350 to about 750 psia; fractionating the pressure adjusted heavyand light hydrocarbon phases to form (a) a methane-nitrogen fractionsubstantially free of carbon dioxide and hydrocarbons heavier thanmethane and (b) a carbon dioxide-hydrocarbon fraction substantially freeof nitrogen and containing C2 and heavier hydrocarbons; fractionatingfurther the carbon dioxide-hydrocarbon fraction to form a C2-C4hydrocarbon fraction and a C4 and heavier hydrocarbon fraction andremoving the C4 and heavier hydrocarbon fraction from the process;combining the C2-C4 hydrocarbon fraction with the carbon dioxide freemethane-nitrogen fraction to form a substantially purified natural gasmixture; cooling and substantially completely liquifying the purifiednatural gas mixture in a countercurrent heat exchanger-liquifier; flashevaporating a minor gas fraction from the cooled effluent from the heatexchanger-liquifier by substantially isenthalpically reducing thepressure thereof, said flash evaporation causing the temperature of boththe minor gas fraction and the remaining major liquid fraction todecrease; collecting the major liquid fraction as product and returningthe flashed fraction to a point where it is combined with anisentropically expanded refrigeration gas fraction produced subsequentlyin the process; passing said combined gases countercurrently to thepurified natural gas mixture in said heat exchanger-liquifier at a flowrate of two to four times that of the purified natural gas stream;separating the warmed effluent from said heat exchanger-liquifier into amajor refrigeration gas fraction and a minor fuel gas fraction; passingthe refrigeration gas fraction through a counter-current heat exchangerprecooler to a compressor system to be compressed to a pressure in therange of about 400 to 600 psia; passing the pressurized gas from saidcompressor system through said heat exchanger precooler to cool therefrigeration gas; isentropically expanding the cooled effluent fromsaid heat exchanger precooler with an expansion engine at a temperatureand pressure sufficient to maintain the gas in the gaseous state in saidexpansion turbine to form the isentropically expanded refrigeration gas,whereby mechanical energy is obtained and the pressure of therefrigeration gas fraction is reduced, thereby further cooling suchfraction; combining the further cooled, reduced pressure, isentropicallyexpanded refrigeration gas effluent from said expansion engine with saidflash gas to a point prior to entry into said heat exchanger-liquifier;and utilizing (a) the fuel gas fraction to furnish fuel for a gas driventurbine which provides a portion of the mechanical energy necessary todrive said compressor system and (b) the mechanical energy obtained fromthe isentropic expansion to provide an additional portion of themechanical energy necessary to drive the said compressor system.
 17. Inan integrated apparatus for purifying and liquifying natural gas, thecombination comprising: gas-liquid separating means for separatingcondensed water and a heavy hydrocarbon condensate from a pressurizednatural gas mixture comprising nitrogen, carbon dioxide, water and lowerboiling hydrocarbons; means for adjusting the pressure of the heavyhydrocarbon condensate and the water and heavy hydrocarboncondensate-free remainder of the natural gas mixture to about 350 toabout 750 psia; means for fractionating the combined, pressure adjustedhydrocarbon condensate and water-free natural gas mixture to form amethane-nitrogen fraction substantially free of carbon dioxide and acarbon dioxide-hydrocarbon fraction substantially free of nitrogen andcontaining predominantly C2 hydrocarbons; means for fractionating thecarbon dioxide-hydrocarbon fraction to form a carbon dioxide fractionand a C2 hydrocarbon fraction, said carbon dioxide fraction includingall of the methane present in the carbon dioxide-hydrocarbon and aportion of the C2 hydrocarbons contained therein; means for combiningthe C2 hydrocarbon fraction with the methane-nitrogen fraction, thusforming a substantially water-free and carbon dioxide-free natural gasmixture; a heat exchanger-liquifier for cooling and liquifying thewater-free and carbon dioxide-free natural gas by countercurrent heatexchange with a combined flash gas and work expanded refrigerant gasfrom an expansion engine; means for isenthalpically expanding anDpartially flashing the natural gas effluent from said heatexchanger-liquifier; gas-liquid separating means for separating saidexpanded effluent into a major liquid fraction and a minor flash gasfraction; means for recovering the major liquid fraction as product;means for passing the flash gas to the heat exchanger-liquifier;compression means for compressing the combined flash gas and workexpanded refrigerant gas from the heat exchanger-liquifier; a heatexchanger precooler for precooling the gas coming from the compressionmeans by countercurrent heat exchange with the combined flash gas andwork expanded gas from said heat exchanger-liquifier; an expansionengine for work expanding said precooled gas, said engine furthercooling the precooled gas and reducing the pressure thereof whilegenerating mechanical energy; means for feeding the work expanded andcooled gas from said expansion engine to said heat exchanger-liquifierto join said flash gas; and means for bleeding from the apparatus aminor portion of the combined flash gas and work expanded gas prior tocompression thereof, said expansion engine and said compression meansbeing operatively cooperative whereby the mechanical energy obtainedfrom said engine supplies at least a portion of the work required tooperate said compression means.
 17. Employing the combined gas mixtureformed in Step (16) to cool the nitrogen-methane and C2-C4 hydrocarbonmixture in Step (13);
 17. Removing from the process system subsequent toStep (8) a minor portion of the combined gas mixture substantially equalin magnitude to the minor methane-nitrogen gas fraction formed in Step(9); and
 18. Employing the mechanical energy obtained from the workrecovery engines of Steps (3) and (16) to compress, at least partly, themajor portion of the combined refrigerant gas mixture in Step (13). 18.Compressing at least a major portion of the heated, combined gas mixturefrom Step (13) to provide a compressed refrigerant gas mixture;
 19. Heatexchanging at least a major portion of the combined gas mixture prior tothe compression of Step (18) with the compressed refrigerant gas mixturewhereby the at least major portion of the heated, combined gas mixtureis further heated to a temperature from about -50* to about 300*F. andthe compressed refrigerant gas mixture is cooled;
 20. Additionallycooling the compressed refrigerant gas to a temperature such that inexpansion in a wOrk recovery engine the temperature and enthalpy aresufficiently low to provide the refrigeration required for Steps (12)and (13);
 21. Substantially isentropically expanding the additionallycooled, compressed refrigerant gas mixture in a work recovery engine toobtain mechanical energy and to further cool the refrigerant gas,whereby the first isentropically expanded, methane-nitrogen refrigerantgas is formed;
 22. Substantially isentropically expanding the heatedfirst isentropically expanded refrigerant gas from Step (12) in a workrecovery engine whereby (a) mechanical energy is obtained, (b) theheated refrigerant gas is cooled, and (c) the second isentropicallyexpanded methane-nitrogen refrigerant gas is formed;
 23. Removing fromthe process system subsequent to Step (13) a minor portion of thecombined gas mixture substantially equal in magnitude to the minormethane-nitrogen gas fraction formed in Step (14); and
 24. Employing themechanical energy obtained from the work recovery engine of Steps (3),(21) and (22) at least to compress partly the major portion of thecombined refrigerant gas mixture in Step (18).